1. Field of Invention
The present invention relates generally to a lithographic apparatus and a device manufacturing method. More specifically, the invention relates to a lithographic apparatus that includes an article handler.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
In lithographic processing, passing the projection beam through gas compositions that are present between the illumination system and the articles to be illuminated, in particular non-homogenous gas compositions, may cause undesired effects, such as diffraction and refraction. These effects may have an adverse effect on illumination quality, in particular on a required resolution to be reached for the ever increasing demand in imaging performance. A new generation of lithography, EUV-lithography, which uses a projection beam in the Extreme Ultraviolet area, operates in (near) vacuum conditions in order to allow the projection beam of radiation to pass substantially unhindered to an article to be placed in the beam.
In the context of this application, the “article” may be a wafer, reticle, mask, or substrate, or more specifically, a substrate to be processed in manufacturing devices employing lithographic projection techniques, or a lithographic projection mask or mask blank in a lithographic projection apparatus, a mask handling apparatus such as mask inspection or cleaning apparatus, or a mask manufacturing apparatus or any other article or optical element that is clamped in the light path of the radiation system.
The above described vacuum technology offers challenges in terms of handling the article. Conventionally, in non-vacuum systems, such article handlers may comprise an ejecting pin mechanism (e-pin) wherein the contact surface of the pin is provided with a vacuum clamp which generates a vacuum suction force during handling of the article. By applying a vacuum force, the article can be clamped tight to the ejection pins, thus providing a reliable connection which guarantees a known position of the wafer during all phases of the handling process.
In vacuum working conditions, a vacuum clamp will no longer be effective and will loose its function. As an alternative to vacuum clamping, a known method of clamping is an electrical clamping method wherein a current is applied from the article handler to the article. In the gap between the handler and the article, an electric attraction force is created which is in the art known as a Johnson-Rahbek (JR) clamping force. Due to a build up of this JR-effect, the method is not very fast and results in delays in clamping which may render handling of the article quite inefficient. Furthermore, the JR-currents may cause thermal expansion effects which may be unfavorable to image resolution.